Magnetic particle, its production method, magnetic recording medium and its production method

ABSTRACT

The present invention relates to a method of producing a magnetic particle including forming a layer containing an alloy particle that can form CuAu type or Cu 3 Au type hard magnetic order alloy phase on a support, oxidizing the layer, and annealing the layer in non-oxidizing atmosphere. The invention also relates to a method of producing a magnetic particle including producing an alloy particle that can form hard magnetic order alloy phase, oxidizing the alloy particle, and annealing the particle in non-oxidizing atmosphere, and a magnetic particle produced by the foregoing production method. Further, the invention relates to a magnetic recording medium comprising a magnetic layer containing a magnetic particle and a method of producing a magnetic recording medium including forming a layer containing an alloy that can form the foregoing hard magnetic order alloy phase, oxidizing the layer, and annealing the layer in non-oxidizing atmosphere.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a magnetic particle and aproduction method thereof as well as a magnetic recording mediumcontaining the magnetic particle in a magnetic layer and a productionmethod thereof.

[0003] 2. Description of the Related Art

[0004] To make the size of a magnetic particle contained in a magneticlayer is required in order to increase the magnetic recording density.For example, with respect to a magnetic recording medium to be widelyused in form of a videotape, a computer tape, a disk or the like, in thecase the weight of a ferromagnetic substance is same, the noisedecreases with decrease in particle size.

[0005] As a hopeful material for the magnetic particle to improve themagnetic recording density, CuAu type or Cu₃Au type hard magnetic orderalloys (e.g. refer to Japanese Patent Application Laid-Open (JP-A) No.2003-73705) can be cited. The hard magnetic order alloys have been knownto have a high crystal magnetic anisotropy due to the strains generatedin an ordering process and show hard magnetism even if the size of themagnetic particle is made small.

[0006] The magnetic particle showing the hard magnetism can be producedby a liquid-phase method and a vapor-phase method and especially, amagnetic particle immediately after the production by a liquid-phasemethod has a disorder phase and a face-centered cubic lattice structure.

[0007] The face-centered cubic lattice thus-generated generally showssoft magnetism or paramagnetism. A magnetic particle having softmagnetism or paramagnetism is not suitable for use as a recording media.In order to obtain a hard magnetic order alloy with a coercive force of95.5 kA/m (1,200 Oe), required for a magnetic recording medium, itrequired to carry out annealing at a temperature not lower than atransformation temperature at which the disorder phase is transformed tothe order phase.

[0008] In the case, the foregoing magnetic particle is produced by aliquid-phase method, the metal composing a magnetic particle is requiredto be annealed in a non-oxidizing atmosphere of such as Ar, N₂, so asnot to be oxidized. However, according to the experiments performed bythe inventors of the invention, when the alloy phase is ordered byannealing, occasionally, the transformation temperature is elevated. Andthe elevated transformation temperature causes problems in the heatresistance of a substrate, the production facilities, and thereproducibility of the magnetic characteristics.

SUMMARY OF THE INVENTION

[0009] As described above, the object of the present invention is toprovide a method of producing a magnetic particle that enables theproduction of a magnetic particle with hard magnetism without increasingthe annealing temperature, and a magnetic particle produced by theproduction method.

[0010] Also, the object of the invention is to provide a magneticrecording medium having a magnetic layer comprising the foregoingmagnetic particle and the production method thereof.

[0011] According to the results of the earnest investigations to solvethe above-mentioned problems, the inventors of the present inventionhave found that the above-mentioned object can be achieved by theinvention described in the following. That is:

[0012] The first aspect of the invention is a method (A1) of producing amagnetic particle including forming on a support a layer containingalloy particles that can form a CuAu type or Cu₃Au type hard magneticorder alloy phase, oxidizing the layer, and annealing the layer in anon-oxidizing atmosphere.

[0013] The second aspect of the invention is the method (A1) ofproducing a magnetic particle, wherein the atmosphere for the annealingis a reducing atmosphere, an annealing temperature is 450° C. or lower,and a retention time is 10 minutes or shorter.

[0014] The third aspect of the invention is the method (A1) of producinga magnetic particle, wherein a third element other than elements thatcan form the CuAu type or Cu₃Au type hard magnetic order alloy phase, isadded to the alloy particle as an alloying element.

[0015] The fourth aspect of the invention is a method (A2) of producinga magnetic particle including producing an alloy particle that can forma CuAu type or Cu₃Au type hard magnetic order alloy phase, oxidizing thealloy particle, and annealing the particle in a non-oxidizingatmosphere.

[0016] The fifth aspect of the invention is the method (A2) of producinga magnetic particle, wherein the atmosphere for the annealing is areducing atmosphere, an annealing temperature is 450° C. or lower, and aretention time is 10 minutes or shorter.

[0017] The sixth aspect of the invention is the method (A2) of producinga magnetic particle, wherein a third element other than elements thatcan form the CuAu type or Cu₃Au type hard magnetic order alloy phase, isadded to the alloy particle as an alloying element.

[0018] The seventh aspect of the invention is a magnetic particle (A3)produced by a method including forming on a support a layer containingalloy particles that can form a CuAu type or Cu₃Au type hard magneticorder alloy phase, oxidizing the layer, and annealing the layer in anon-oxidizing atmosphere.

[0019] The eighth aspect of the invention is the magnetic particle (A3),wherein the atmosphere for the annealing is a reducing atmosphere, anannealing temperature is 450° C. or lower, and a retention time is 10minutes or shorter.

[0020] The ninth aspect of the invention is the magnetic particle (A3),wherein a third element other than elements that can form the CuAu typeor Cu₃Au type hard magnetic order alloy phase, is added to the alloyparticles as an alloying element.

[0021] The tenth aspect of the invention is a magnetic particle (A4)produced by a method including producing an alloy particle that can forma CuAu type or Cu₃Au type hard magnetic order alloy phase, oxidizing thealloy particle, and annealing the particle in a non-oxidizingatmosphere.

[0022] The eleventh aspect of the invention is the magnetic particle(A4), wherein the atmosphere for the annealing is a reducing atmosphere,an annealing temperature is 450° C. or lower, and a retention time is 10minutes or shorter.

[0023] The twelfth aspect of the invention is the magnetic particle(A4), wherein a third element other than elements that can form the CuAutype or Cu₃Au type hard magnetic order alloy phase, is added to thealloy particle as an alloying element.

[0024] The thirteenth aspect of the invention is the magnetic recordingmedium (A5) comprising a magnetic layer containing magnetic particlesproduced by a method including forming on a support a layer containingalloy particles that can form a CuAu type or Cu₃Au type hard magneticorder alloy phase, oxidizing the layer, and annealing the layer in anon-oxidizing atmosphere.

[0025] The fourteenth aspect of the invention is the magnetic recordingmedium (A5), wherein the atmosphere for the annealing is a reducingatmosphere, an annealing temperature is 450° C. or lower, and aretention time is 10 minutes or shorter.

[0026] The fifteenth aspect of the invention is the magnetic recordingmedium (A5), wherein a third element other than elements that can formthe CuAu type or Cu₃Au type hard magnetic order alloy phase, is added tothe alloy particles as an alloying element.

[0027] The sixteenth aspect of the invention is a magnetic recordingmedium (A6) comprising a magnetic layer containing magnetic particlesproduced by a method including producing alloy particles that can form aCuAu type or Cu₃Au type hard magnetic order alloy phase, oxidizing thealloy particles, and annealing the particles in non-oxidizingatmosphere.

[0028] The seventeenth aspect of the invention is the magnetic recordingmedium (A6), wherein the atmosphere for the annealing is a reducingatmosphere, an annealing temperature is 450° C. or lower, and aretention time is 10 minutes or shorter.

[0029] The eighteenth aspect of the invention is the magnetic recordingmedium (A6), wherein a third element other than elements that can formthe CuAu type or Cu₃Au type hard magnetic order alloy phase, is added tothe alloy particles as an alloying element.

[0030] The ninteenth aspect of the invention is a method (A7) ofproducing a magnetic recording medium including forming on a support alayer containing an alloy that can form a CuAu type or Cu₃Au type hardmagnetic order alloy phase, oxidizing the layer, and annealing the layerin a non-oxidizing atmosphere.

[0031] The twentieth aspect of the invention is the method (A7) ofproducing a magnetic recording medium according to claim 19, wherein theatmosphere for the annealing is a reducing atmosphere, an annealingtemperature is 450° C. or lower, and a retention time is 10 minutes orshorter.

DETAILED DESCRIPTION OF THE INVENTION

[0032] <<Magnetic Particle and its Production Method>>

[0033] The first method of producing the magnetic particle according tothe present invention includes an alloy particle production step, inwhich an alloy particle that can form hard magnetic order alloy phase isproduced by a liquid phase method or a vapor phase method; oxidationstep, in which the produced alloy particle is oxidized; and annealingstep, in which the alloy particle is annealed in a non-oxidizingatmosphere after oxidation.

[0034] Hereinafter, a method of producing a magnetic particle andmagnetic particle of the invention will be described along withdescriptions of the foregoing respective steps.

[0035] <Alloy Particle Production Step>

[0036] An alloy particle that can be converted to a magnetic particle byannealing can be produced by a vapor phase method or a liquid phasemethod. In consideration of suitability for mass production, the liquidphase method is preferable. As the liquid phase method, a variety ofconventionally known methods can be applied. A reducing method, which isan improvement of the conventional method, is preferably employed and,among them, a reverse micelle method by which the particle size can beeasily controlled is especially preferable.

[0037] (Reverse Micelle Method)

[0038] The reverse micelle method includes at least (1) a reduction stepin which reduction reaction is proceeded by mixing two types of reversemicelle solutions and (2) an aging step in which aging is proceeded at aprescribed temperature after the reduction.

[0039] Hereinafter, the respective steps will be described.

[0040] (1) Reduction Step:

[0041] At first, a reverse micelle solution (I) is prepared by mixing awater-insoluble organic solvent containing a surfactant and an aqueousreducing agent solution.

[0042] As the foregoing surfactant, an oil-soluble surfactant is used.Specifically, sulfonate types (e.g. Aerosol OT (produced by Wako PureChemical Industries, Ltd.), quaternary ammonium salt types (e.g.cetyltrimethylammonium bromide), ether types (e.g. pentaethyleneglycoldodecyl ether) and the like can be exemplified.

[0043] The amount of the surfactant included in the water-insolubleorganic solvent is preferably 20 to 200 g/l.

[0044] Preferable water-insoluble organic solvents to dissolve thesurfactant are alkanes, ethers and alcohols.

[0045] As alkanes, alkanes having 7 to 12 carbons are preferable.Specifically, heptane, octane, isooctane, nonane, decane, undecane,dodecane and the like are preferable.

[0046] As ethers, diethyl ether, dipropyl ether, dibutyl ether and thelike are preferable.

[0047] As alcohols, ethoxyethanol, ethoxypropanol and the like arepreferable.

[0048] As the reducing agent included in an aqueous reducing agentsolution, alcohols; polyalcohols; H₂; compounds containing HCHO, S₂O₆⁻², H₂PO₂ ⁻, BH₄ ⁻, N₂H₅ ⁺, H₂PO₃ ⁻, and the like are preferably usedalone or in combination with two or more types of them.

[0049] The amount of the reducing agent in the aqueous solution ispreferably 3 to 50 mole with respect to 1 mole of a metal salt.

[0050] Here, the mass ratio (water/surfactant) of water and thesurfactant in the reverse micelle solution (I) is preferably 20 orlower. If the mass ratio exceeds 20, a precipitation tends to be formedand the particles tend to become uneven. The mass ratio is adjusted tobe preferably 15 or lower, more preferably 0.5 to 10.

[0051] Besides, a reverse micelle solution (II) is prepared by mixing awater-insoluble organic solvent containing a surfactant and an aqueousmetal salt solution.

[0052] The conditions (the substance to be used, the concentration, andthe like) of the surfactant and the water-insoluble organic solvent aresimilar to those in the case of the reverse micelle solution (I).

[0053] Incidentally, the same solution as or a solution different fromthe reverse micelle solution (I) can be used. Further, the mass ratio ofwater and the surfactant in the reverse micelle solution (II) may bewithin the same range as that in the reverse micelle solution (I) andmay be adjusted to be the same value as or a value different from thatof the reverse micelle solution (I).

[0054] As the metal salt contained in the aqueous metal salt solution,it is preferable to select a proper metal salt so as to make a magneticparticle possible to form CuAu type or Cu₃Au type ferromagnetic orderalloy.

[0055] Here, as the CuAu type ferromagnetic order alloy, FeNi, FePd,FePt, CoPt, CoAu and the like can be exemplified and among them,preferable are FePd, FePt, and CoPt.

[0056] As the Cu₃Au type ferromagnetic order alloy, Ni₃Fe, FePd₃, Fe₃Pt,FePt₃, CoPt₃, Ni₃Pt, CrPt₃, Ni₃Mn can be exemplified and among them,preferable are FePd₃, FePt₃, CoPt₃, Fe₃Pd, Fe₃Pt, and Co₃Pt.

[0057] Specific examples of the metal salt include H₂PtCl₆, K₂PtCl₄,Pt(CH₃COCHCOCH₃)₂, Na₂PdCl₄, Pd(OCOCH₃)₂, PdCl₂, Pd(CH₃COCHCOCH₃)₂,HAuCl₄, Fe₂(SO₄)₃, Fe(NO₃)₃, (NH₄)₃Fe(C₂H₄)₃, Fe(CH₃COCHCOCH₃)₃, NiSO₄,CoCl₂, Co(OCOCH₃)₂ and the like.

[0058] The concentration of the aqueous metal salt solution (as themetal salt concentration) is preferably 0.1 to 1,000 μmol/ml, morepreferably 1 to 100 μmol/ml.

[0059] Proper selection of the foregoing metal salt makes it possible toproduce an alloy particle that can form the CuAu type or Cu₃Au typeferromagnetic order alloy in which a base metal and a noble metal arealloyed.

[0060] It is required for the alloy particle to transform the alloyphase from the disorder phase to the order phase by annealing and inorder to lower the transformation temperature. It is preferable to addthe third element such as Sb, Pb, Bi, Cu, Ag, Zn, and In to theforegoing binary alloys. It is preferable to add previously precursorsof the respective third elements to the metal salt solution. Theaddition amount is preferably 1 to 30 at %, more preferably 5 to 20 at %to the binary alloys.

[0061] The reverse micelle solutions (I) and (II) prepared in such amanner are mixed. The mixing method is not particularly limited, howevertaking the uniformity of reduction into consideration, it is preferableto carry out mixing by adding the reverse micelle solution (II) whilestirring the reverse micelle solution (I). On completion of the mixing,the reduction is going to be caused and at that time, the temperature ispreferably constant within a range from −5 to 30° C.

[0062] If the reduction temperature is lower than −5° C., problems suchas freezing of the water phase that causes uneven reduction occur, andif it exceeds 30° C., flocculation or precipitation easily takes placeand the reaction system becomes unstable in some cases. The reductiontemperature is preferably 0 to 25° C., more preferably 5 to 25° C.

[0063] Here, the above-mentioned “constant temperature” means that thetemperature is within the range of ±3° C., wherein the set temperatureis defined as T (° C.). And the upper limit and the lower limit of thereducing temperature are still within the above-mentioned range of thetemperature (−5 to 30° C.).

[0064] The duration of the reduction should be properly set depending onthe amounts or the like of the reverse micelle solutions and ispreferably 1 to 30 minutes, more preferably, 5 to 20 minutes.

[0065] It is preferable for the reduction to be carried out under higherspeed stirring condition possible, since the process of the reductionsignificantly affects monodispersion of the particle size distribution.

[0066] A preferable stirring apparatus is a stirring apparatus having ahigh shearing force and specifically, the stirring apparatus is those inwhich stirring blades basically have a turbine type or a paddle typestructure and further, the sharp edge are attached to the ends of theblades or the positions contacting the stirring blades, and the bladesare rotated by a motor. More specifically, Dissolver (manufactured byTokushu Kika Kogyo Co., Ltd.), Omnimixer (manufactured by YamatoScientific Co., Ltd.), Homogenizer (manufactured by SMT) and the likeare useful. By employing these apparatuses, type alloy particles havingmonodispersion distribution can be obtained in form of a stabledispersion.

[0067] It is preferable to add at least one kind of dispersants having 1to 3 amino groups or carboxyl groups to at least one of the foregoingreverse micelle solutions (I) and (II) in an amount of 0.001 to 10 moleper 1 mole of the alloy particle to be produced.

[0068] Addition of such a dispersant makes it possible to obtain alloyparticles free from flocculation having monodispersion distribution.

[0069] If the addition amount is less than 0.001 mole, themonodispersion property of the alloy particle cannot be improved in somecases and if it exceeds 10 mole, flocculation sometimes takes place.

[0070] As the foregoing dispersant, an organic compound having a groupadhering to the surface of the alloy particle is preferable.Specifically, an organic compound having 1 to 3 groups selected from agroup of amino groups, carboxyl groups, sulfonic acid groups, andsulfinic acid groups, are employed. They may be used alone or incombination of them.

[0071] The compound can be represented by a structural formula; R—NH₂,NH₂—R—NH₂, NH₂—R(NH₂)—NH₂, R—COOH, COOH—R—COOH, COOH—R(COOH)—COOH,R—SO₃H, SO₃H—R—SO₃H, SO₃H—R(SO₃H)—SO₃H, R—SO₂H, SO₂H—R—SO₂H,SO₂H—R(SO₂H)—SO₂H wherein R denotes a linear, branched or cyclicsaturated or unsaturated hydrocarbon.

[0072] A compound especially preferable as a dispersant is oleic acid.The oleic acid is a well-known surfactant for stabilizing colloids andhas been used for protecting metal particles of such as iron or thelike. Relatively long chain of the oleic acid (for example, oleic acidhas a chain of 18 carbons with a length of about 20 Å (about 2 nm).Oleic acid is not an aliphatic compound but has one double bond) givesimportant steric hindrance canceling mutual magnetic reaction amongparticles.

[0073] In the same manner as the case of oleic acid, similar long chaincarboxylic acids such as erucic acid, linoleic acid and the like (forexample long chain organic acids containing 8 to 22 carbon atoms can beused alone or in combination) can be used. Oleic acid is an economicalnatural resource that is easily available (from olive oil or the like).Therefore, it is preferable. Oleylamine derived from the oleic acid isalso a usable dispersant as well as oleic acid.

[0074] In the reduction step as described above, it is considered thatmetals with a lower redox potential [metals with about −0.2 V (vs. N. H.E) or lower] such as Co, Fe, Ni, Cr or the like to be contained in theCuAu type or Cu₃Au type hard magnetic order alloy phase are reduced andprecipitated in form of a particle having a monodispersion distributionand a minimum size. After that, in the temperature elevation step and anaging step to be described later, the precipitated base metal becomes acore and on its surface, metals with a higher redox potential [metalswith about −0.2 V (vs. N. H. E) or higher] such as Pt, Pd, Rh and thelike are reduced by the base metals and precipitated, replacing the basemetals. It is supposed that the ionized base metals are reduced again bya reducing agent and precipitated. Such steps are repeated and an alloyparticle that can form the CuAu type or Cu₃Au type hard magnetic orderalloy can be obtained.

[0075] (2) Aging Step:

[0076] On completion of the reduction, the solution after the reactionis heated to an aging temperature.

[0077] The foregoing aging temperature is preferably controlled at aconstant temperature, which is higher than the temperature in theforegoing reduction and in the range of 30 to 90° C. The duration ofaging is preferably 5 to 180 minutes. If the aging temperature is higherthan the foregoing range or duration is longer than the foregoing range,flocculation and precipitation easily take place. On the contrary, ifthe temperature is lower than the foregoing range or the duration isshorter than the foregoing range, occasionally, the reaction can not becompleted and, as a result, the composition of the alloy changes. Thepreferable aging temperature is 40 to 80° C., more preferably, 40 to 70°C. And preferable duration is 10 to 150 minutes, more preferably 20 to120 minutes.

[0078] Here, the foregoing “constant temperature” means the same as thecase of the temperature at the reduction (in this case, “the reductiontemperature” is replaced with “the aging temperature”). Especially, theaging temperature is preferably at least 5° C., more preferably at least10° C., higher than the reduction temperature, still being kept in therange of the foregoing aging temperature (30 to 90° C.). If the agingtemperature is lower than the temperature 5° C. higher than thereduction temperature, it sometimes becomes impossible to obtain aprescribed composition.

[0079] In the aging step as described above, noble metals areprecipitated on the base metals reduced and precipitated in thereduction step.

[0080] That is, reduction of the noble metals takes place only on thebase metals and it does not occur that the base metals and noble metalsare separately precipitated. Thus, an alloy particle that can form theCuAu type or Cu₃Au type hard magnetic order alloy can be produced at ahigh yield with a prescribed composition ratio, and the composition canbe controlled as desired. Also, by properly adjusting the stirring speedat the time of aging, the diameter of the alloy particle to be obtainedcan be controlled as desired.

[0081] After the foregoing aging, a washing and dispersing step ispreferably carried out, in which the solution after the foregoing agingis washed with a mixed solution of water and a primary alcohol and afterthat, a precipitate is formed from the solution by a precipitationtreatment with a primary alcohol, and dispersing the precipitate with anorganic solvent.

[0082] By performing the washing and dispersing step, impurities areremoved and the coating property in the case of formation of a magneticlayer of a magnetic recording medium by coating can be improved.

[0083] The foregoing washing and dispersing step may be carried out atleast once, preferably twice, respectively.

[0084] The foregoing primary alcohol employed for washing is notparticularly limited. And methanol, ethanol or the like is preferable.The mixing ratio by volume (water/primary alcohol) is preferably in therange of 10/1 to 2/1, more preferably in the range of 5/1 to 3/1.

[0085] If the water ratio is high, it becomes difficult to remove thesurfactant in some cases and on the contrary, if the ratio of theprimary alcohol is high, flocculation sometimes occurs.

[0086] As described above, the alloy particle dispersed in the solution(an alloy particle-containing solution) can be obtained.

[0087] Since the alloy particles have monodispersion distribution, evenif it is applied to a support, the alloy particles are not flocculatedand maintain the uniformly dispersed state. Accordingly, the respectiveparticles are not flocculated even if annealing treatment is carriedout, the alloy particle can be efficiently given hard magnetism and isexcellent in coating suitability.

[0088] The diameter of the alloy particle before the oxidation, whichwill be described later, is preferably small in terms of suppression ofnoise, however if it is too small, the particle occasionally becomessuperparamagnetic after annealing and becomes unsuitable for use inmagnetic recording. Generally the diameter of the alloy particle ispreferably 1 to 100 nm, more preferably 1 to 20 nm, further preferably 3to 10 nm.

[0089] (Reduction Method)

[0090] There are a variety of methods for producing the alloy particlethat can form a CuAu type or Cu₃Au type hard magnetic order alloy byreduction. A method is preferable in which a metal with a lower redoxpotential (hereinafter, simply referred to as “a base metal” in somecases) and a metal with a higher redox potential (hereinafter, simplyreferred to as “a noble metal”) are reduced with a reducing agent or thelike in an organic solvent, water, or a mixed solution of an organicsolvent and water.

[0091] The sequence of the reduction of the base metal and the noblemetal is not particularly limited and both may be simultaneouslyreduced.

[0092] As the foregoing organic solvent, alcohol, polyalcohol and thelike can be used and methanol, ethanol, butanol and the like can becited as the alcohol and ethylene glycol, glycerin and the like can becited as the polyalcohol.

[0093] Examples of the CuAu type or Cu₃Au type hard magnetic order alloyare the same as exemplified in the above-mentioned reverse micellemethod.

[0094] Also, as a method for producing an alloy particle byprecipitating the noble metal before the base metal, a method disclosedin paragraph 18 to 30 in Japanese Patent Application No. 2001-269255 canbe employed.

[0095] As the metal with a higher redox potential, Pt, Pd, Rh and thelike are preferable to be employed and H₂PtCl₂.6H₂O, Pt(CH₃COCHCOCH₃)₂,RhCl₃.3H₂O, Pd(OCOCH₃)₂, PdCl₂, Pd(CH₃COCHCOCH₃)₂ and the like can beused while being dissolved in a solvent. The concentration of the metalin a solution is preferably 0.1 to 1,000 μmol/ml, more preferably 0.1 to100 μmol/ml.

[0096] As the metal with a lower redox potential, Co, Fe, Ni, and Cr arepreferable to be employed and especially preferable one is Fe and Co. Assuch a metal, FeSO₄.7H₂O, NiSO₄.7H₂O, CoCl₂.6H₂O, Co(OCOCH₃)₂.4H₂O canbe used by dissolving them in a solvent. The concentration of the metalin a solution is preferably 1 to 1,000 μmol/ml, more preferably 0.1 to100 μmol/ml.

[0097] Further, similarly to the case of employing foregoing reversemicelle method, it is preferable to lower the transformation temperatureto the hard magnetic order alloy by adding the third element to a binaryalloy. The addition amount is same as that in the case of the reversemicelle method.

[0098] For example, in a case a base metal and a noble metal aresuccessively reduced in this order by using a reducing agent, it ispreferable to carry out the reduction as follows: the base metal or thebase metal with a portion of the noble metal reduced with a reducingagent having a reduction potential lower than −0.2 V (vs. N.H.E) isadded to a noble metal source and reducing the mixture with a reducingagent having a redox potential higher than −0.2 V (vs. N.H.E), and afterthat, reducing the mixture with a reducing agent having a reductionpotential lower than −0.2 V (vs. N.H.E).

[0099] Although the redox potential depends on the pH in the reactionsystem, as the reducing agent with a redox potential higher than −0.2 V(vs. N.H.E), alcohols such as 1,2-hexadecanediol; glycerin compounds;H₂; and HCHO are preferably used.

[0100] As the reducing agent with a redox potential lower than −0.2 V(vs. N.H.E), S₂O₆ ²⁻, H₂PO₂ ⁻, BH₄ ⁻, N₂H₅ ⁺, and H₂PO₃ ⁻ are preferablyused.

[0101] Here, in the case a 0 valent metal compound such as Fe carbonylis used as a raw material of the base metal, a reducing agent is notparticularly required.

[0102] In reduction precipitation of the noble metal, an alloy particlecan be stably produced in the presence of an adsorbent. As theadsorbent, a polymer and a surfactant can be preferably used.

[0103] As the foregoing polymer, polyvinyl alcohol (PVA),poly(N-vinyl-2-pyrrolidone) (PVP), gelatin and the like can beexemplified. Among them, especially preferable one is PVP.

[0104] The molecular weight of the polymer is preferably 20,000 to60,000, more preferably 30,000 to 50,000. The amount of the polymer ispreferably 0.1 to 10 times, more preferably 0.1 to 5 times, the weightof the alloy particles to be produced.

[0105] The surfactant preferably used as the adsorbent preferablycontains “an organic stabilizer”, which is a long chain organic compoundrepresented by the general formula: R—X. In the formula, R denotes “atale group”, which is a linear or branched hydrocarbon or fluorocarbonchain and generally contains 8 to 22 carbon atoms. And X represents “ahead group”, which is a portion (X) giving a specified chemical bond tothe surface of the alloy particle and preferably selected from the groupof sulfinate (—SOOH), sulfonate (—SO₂OH), phosphinate (—POOH),phosphonate (—OPO(OH)₂), carboxylate, and thiol.

[0106] The foregoing organic stabilizer is preferably selected from thegroup of sulfonic acid (R—SO₂OH), sulfinic acid (R—SOOH), phosphinicacid (R₂POOH), phosphonic acid (R—OPO(OH)₂), carboxylic acid (R—COOH),and thiol (R—SH). Among them, similarly to the reverse micelle method,oleic acid is especially preferable.

[0107] The combination of the foregoing phosphine and the organicstabilizer (e.g. triorganophosphine/acid) can provide excellentcontrollability to the growth and stabilization of the particle.Although didecyl ether and didodecyl ether can be used, phenyl ether andn-octyl ether can be used preferably as a solvent due to the low costand high boiling point of them.

[0108] The reaction is carried out preferably in the range of 80° C. to360° C., and more preferably in the range of 80° C. to 240° C.,depending on the required alloy particle and the boiling point of thesolvent. The particle does not grow if the temperature is lower than thetemperature range in some cases. If the temperature is higher than therange, the particle grows without control and undesirable by-productsmay grow in some cases.

[0109] Similarly to that in the reverse micelle method, the particlesize of the alloy particle is preferably 1 to 100 nm, more preferably 3to 20 nm, and further preferably 3 to 10 nm.

[0110] A seed crystallization method is effective as the method forincreasing the particle size (the particle diameter). In the case thealloy particle is used as a magnetic recording medium, it is preferableto pack the alloy particle in the closest packing state in order toincrease the recording capacity. For achieving the closest packingt, thestandard deviation of the size of the alloy particle is preferably lessthan 10%, more preferably 5% or less. The variation coefficient of theparticle size is preferably less than 10%, more preferably 5% or less.

[0111] If the particle size is too small, the alloy particle becomessuper-paramagnetic and this phenomenon is not preferable. Therefore, inorder to enlarge the particle size, the seed crystallization method ispreferable as described above. In the process of the seedcrystallization, a metal having higher redox potential than the metalscomposing the particle sometimes precipitates. In such a case, oxidationof the particle is considered to occur. Accordingly, the particle ispreferably hydrogenated prior to the seed crystallization.

[0112] It is preferable for a noble metal to form an outermost layer ofthe alloy particle from a viewpoint of oxidation prevention. However,particle having an outermost layer composed of a noble metal flocculateeasily. According to the invention, an alloy of a noble metal and a basemetal is preferable for the layer. Such a constitution can be formedeasily and efficiently by a liquid-phase method.

[0113] Removal of salts from the solution after the alloy particlesynthesis is preferable in terms of improvement of the dispersionstability of the alloy particle. To remove the salts, an alcohol isadded excessively to cause slight flocculation, spontaneously orcentrifugally cause precipitation, and remove the salts together withthe supernatant solution. However, such a method easily causesflocculation. Hence, an ultra filtration method is preferable to beemployed. Thus, the alloy particle dispersed in a solution (an alloyparticle-containing solution) can be obtained.

[0114] A transmission electron microscope (TEM) may be employed for theparticle size evaluation of the alloy particle. Although electrondiffraction by TEM can be employed to determine crystal system of thealloy particle or the magnetic particle, x-ray diffraction is preferablyemployed since it has a high precision. For the composition analysis ofthe inside of the alloy particle or the magnetic particle, FE-TEMequipped with EDAX which emits a convergent electron beam, may bepreferably employed for the evaluation. Further, the evaluation of themagnetic property of the alloy particle or the magnetic particle can becarried out using VSM.

[0115] <Oxidation Step>

[0116] By oxidizing thus-obtained alloy particle, a magnetic particlewith hard magnetism can be efficiently produced without elevating thetemperature at annealing in a non-oxidizing atmosphere thereafter. Thatis supposedly attributed to the phenomenon described as follows.

[0117] That is, at first, oxygen enters in the crystal lattice byoxidizing the alloy particle. When annealing is carried out withinvasion of the oxygen in the lattice, oxygen is dissociated from thecrystal lattice by the heat. Defects are generated by the dissociationof oxygen and since the metal atoms composing the alloy easilytranslocate through the defects, phase transformation is supposedlycaused easily even at a relatively low temperature.

[0118] Such a phenomenon can be supported by measuring the EXAFS(expanded range x-ray absorption fine structure) of the alloy particleafter the oxidation and the magnetic particle subjected to theannealing.

[0119] For example, in a Fe—Pt alloy particle not experiencing theoxidizing treatment, a bond between a Fe atom and a Pt atom or a Fe atomcan be confirmed.

[0120] On the contrary, in an alloy particle experiencing the oxidationtreatment, a bond between a Fe atom and an oxygen atom can be confirmed.On the other hand, a bond between a Fe atom and a Pt atom and a Fe atomare scarcely observed. That means the bonds of Fe—Pt, Fe—Fe are cut byoxygen atoms. Accordingly it is supposed that Pt atoms and Fe atomsbecome easy to move at annealing.

[0121] Then, after the alloy particle is annealed, existence of oxygencannot be confirmed and existence of bonds between a Fe atom and a Ptatoms or a Fe atom in the surrounding of a Fe atom can be confirmed.

[0122] Taking the above-mentioned phenomenon into consideration, it canbe understood that the phase transformation is difficult to proceedwithout oxidation and the annealing temperature is required to be highwithout oxidation. However, if oxidation is carried out to an excessextent, the mutual reaction between oxygen and a metals that is easy tobe oxidized such as Fe becomes so intense as to produce a metal oxide.

[0123] Accordingly, control of the oxidation state of the alloy particlebecomes important and therefore, it is required to proceed the oxidationat the optimistic condition.

[0124] The oxidation can be carried out, for example, in the case ofproduction of the alloy particle by the liquid phase method as describedabove, by supplying a gas containing at least oxygen to the producedalloy particle-containing solution.

[0125] The partial pressure of the oxygen is preferably 10 to 100%, morepreferably 15 to 50%, of the total pressure.

[0126] The oxidation temperature is preferably 0 to 100° C., morepreferably 15 to 80° C.

[0127] The oxidation state of the alloy particle is preferably evaluatedby EXAFS and the like. The number of bonds of a base metal such as Fewith oxygen is preferably 0.5 to 4, more preferably 1 to 3, from aviewpoint of cutting the Fe—Fe bonds and Pt—Fe bonds by oxygen.

[0128] Further, the foregoing alloy particle can be oxidized beingcoated or fixed on a support by exposure to the air at a roomtemperature (0 to 40° C.). Oxidation of the alloy particle being coatedon a support prevents the flocculation of the alloy particle. Theduration of the oxidation is preferably 1 to 48 hours, more preferably 3to 24 hours.

[0129] <Annealing Treatment>

[0130] The alloy particle after the oxidation is in disorder phase. Thealloy particle in a disorder phase can not attain hard magnetization asdescribed above. Therefore, in order to convert the phase of the alloyparticle to the order phase, a heating treatment (annealing) is requiredto be conducted on the alloy particle. The transformation temperature,at which the alloy composing the alloy particle order-disordertransforms, can be obtained by using a differential thermal analyzer(DTA). It is required to carry out the heating treatment at atemperature equal to or higher than the transformation temperature.

[0131] The foregoing transformation temperature is generally about 500°C., however it is sometimes decreased by addition of the third element.Further, the transformation temperature can be decreased by changing theatmosphere at the above-mentioned oxidation and annealing properly.Accordingly, the annealing temperature is preferably adjusted to be 150°C. or higher, more preferably 150 to 450° C.

[0132] Representative magnetic recording media are a magnetic recordingtape and a floppy (R) disk. They are produced by forming a magneticlayer in web state on a support, which is composed of an organicsubstance, and then processing the resultant substrate into a tape-statefor the former and punching the substrate into a disk-state for thelatter. The invention is effective in the case an organic support isused since the transformation temperature to the ferromagnetism can belowered. Thus, the invention can preferably applied to such mediums.

[0133] When annealing the alloy in a web state, annealing duration ispreferably short. That is because if the annealing duration is long, theapparatus becomes very large and long. For example, in the case theannealing duration is set at 30 minutes and the transportation speed ofa web is set at 50 m/min, the line length becomes as long as 1,500 m.Therefore, in a method of producing a magnetic particle of theinvention, the annealing duration is preferably 10 minutes or shorter,more preferably 5 minutes or shorter.

[0134] In order to shorten the annealing duration as described above,the annealing is preferably proceeded at a reducing atmosphere asdescribed later. Shortening of the annealing duration is effective inprevention of a deformation of the support and a diffusion of impuritiesfrom the support.

[0135] If the alloy is annealed in a particle state, the particle easilymoves to cause fusion of the particles. Therefore, although a highcoercive force can be obtained, the resultant magnetic recording mediumtends to have a disadvantage that the particle size becomes large.Accordingly, the alloy particle is preferably annealed being coated on asupport or the like in terms of prevention of flocculation of the alloyparticle.

[0136] Further, by annealing an alloy particle on a support to give amagnetic particle, a magnetic recording medium comprising a magneticlayer containing such a magnetic particle can be obtained.

[0137] As the support, both inorganic and organic supports can be usedas long as they are usable for a magnetic recording medium.

[0138] As a support of an inorganic material, Al, an Mg alloy such asAl—Mg, Mg—Al—Zn and the like, glass, quartz, carbon, silicon, ceramicand the like can be employed. Those supports are excellent in impactresistance and have rigidity suitable for thinning and high-speedrotation. Further, as compared with a support of an organic material,they are more resistant to heat.

[0139] As a support of an organic material, polyesters such aspolyethylene terephthalate, polyethylene naphthalate; polyolefins;cellulose triacetate; polycarbonate; polyamide (including aliphaticpolyamide and aromatic polyamide such as aramide); polyimide;polyamideimide; polysulfone; polybenzoxazole; and the like can beemployed.

[0140] To coat the alloy particle on a support, a variety of additivesare added, if necessary, to a solution containing an alloy particleafter the foregoing oxidation and the mixture is coated on a support.

[0141] The content of the alloy particle is preferably a desiredconcentration in a range of 0.01 to 0.1 mg/ml.

[0142] As a method for coating the alloy particle on a support, airdoctor coat, blade coat, rod coat, extrusion coat, air knife coat,squeezing coat, impregnation coat, reverse roll coat, transfer rollcoat, gravure coat, kiss coat, cast coat, spray coat, spin coat, and thelike can be employed.

[0143] The atmosphere at annealing should be a non-oxidizing atmosphereof H₂, N₂, Ar, He, Ne and the like in order to efficiently promote phasetransformation and prevent oxidation of the alloy.

[0144] Particularly, in terms of dissociation of oxygen having enteredin the lattice by oxidation, the annealing is conducted preferably in areducing atmosphere of such as methane, ethane, H₂, and the like.Further, in terms of particle diameter retention, annealing ispreferably carried out in a magnetic field under the reducingatmosphere. Incidentally, in the case H₂ atmosphere is employed, aninert gas is preferably added in terms of prevention of explosion.

[0145] Further, in order to prevent fusion of the particle at annealing,it is preferable to carry out annealing once at a temperature equal toor lower than the transformation temperature in an inert gas tocarbonize the dispersant and then carry out annealing at a temperatureequal to or higher than the transformation temperature in a reducingatmosphere. In this case, the most preferable embodiment is that afterthe foregoing annealing is carried out at a temperature equal to orlower than the transformation temperature, depending on the necessity, aSi-type resin or the like is coated on the layer of the alloy particleand then the annealing is carried out at a temperature equal to orhigher than the transformation temperature.

[0146] By carrying out such annealing as described above, the alloyparticle is transformed from the disorder phase to order phase and amagnetic particle exhibiting hard magnetism can be obtained.

[0147] A magnetic particle produced by the above-mentioned method ofproducing a magnetic particle of the invention preferably has a coerciveforce of 95.5 to 398 kA/m (1,200 to 5,000 Oe). And in the case it isapplied to a magnetic recording medium, it more preferably has acoercive force of 95.5 to 278.6 kA/m (1,200 to 3,500 Oe) inconsideration of the compatibility of a recording head.

[0148] Further, the particle diameter of the magnetic particle ispreferably 1 to 100 nm, more preferably 3 to 20 nm, and furtherpreferably 3 to 10 nm.

[0149] A second method of producing magnetic particle according to theinvention includes steps of forming a layer containing an alloy particlethat can form CuAu type or Cu₃Au type hard magnetic order alloy phase ona support, subjecting it to oxidation treatment, and then annealing itin non-oxidizing atmosphere.

[0150] The production method includes some common points with theabove-mentioned first method of producing a magnetic particle, howeverit differs from the first method in a point that the foregoing alloyparticle is produced by being directly precipitated on a support andsubjected to oxidation and annealing treatment.

[0151] As the foregoing precipitation method, any method which canprecipitate a desired alloy particle on a support and form a layercontaining the alloy particle can be employed without limitation. Asputtering film formation method is preferable for the production.

[0152] The sputtering film formation method includes “RF magnetronsputtering method (hereinafter, sometimes referred to as “RF sputteringmethod”), “DC magnetron sputtering method”, and the like and any of themcan be employed. The “RF sputtering method is preferable since it canefficiently form an alloy particle that can form CuAu type or Cu₃Au typehard magnetic order alloy phase.

[0153] Segregation of Si, Cr or the like in crystal grain boundaries ispreferable to lower the magnetization unit and suppress the noise.

[0154] The CuAu type or Cu₃Au type order alloy film formed by sputteringis paramagnetic or soft magnetic and becomes hard magnetic by annealing.In this case, according to the invention, annealing in non-oxidizingatmosphere, preferably in reducing atmosphere, after oxidation iseffective in terms of lowering the transformation temperature.

[0155] The oxidation after the film formation is preferably conducted bya method similar to the first production method, in which oxidation iscarried out by exposure to the air.

[0156] After the oxidation is carried out, the alloy particle isannealed under the same conditions as those of the first productionmethod to obtain a magnetic particle having hard magnetism.

[0157] <<Magnetic Recording Medium>>

[0158] A magnetic recording medium of the invention comprises a magneticlayer containing a magnetic particle which is produced by the method ofproducing a magnetic particle of the invention described above.

[0159] The magnetic recording medium includes a magnetic tape such as avideo tape, a computer tape and the like; a magnetic disk such as afloppy (R) disk, a hard disk and the like.

[0160] In the case an alloy particle (an alloy particle-containingsolution) is coated on a support and annealed to obtain a magneticparticle as described above, a layer containing such a magnetic particlecan be a magnetic layer.

[0161] Further, in the case the alloy particle is annealed in a particlestate rather than annealed while being coated on a support to produce amagnetic particle, a coating solution is prepared by kneading themagnetic particle by an open kneader, three-roll mill and the like andthen finely dispersing the magnetic particle by a sand grinder or thelike and then the coating solution is coated on a support by a knownmethod to form a magnetic layer.

[0162] Further as described in “the second method of producing amagnetic particle according to the invention”, the magnetic recordingmedium may be produced by forming a layer containing an alloy that canform CuAu type or Cu₃Au type hard magnetic order alloy phase on asupport by the sputtering film formation method, oxidizing it, andannealing it in non-oxidizing atmosphere to form a magnetic layer.

[0163] In this case, the oxidation can be performed by theabove-mentioned exposure to air at a room temperature (0 to 40° C.).Further, the annealing is preferable to be carried out in the manner asdescribed in “the first method of producing a magnetic particleaccording to the present invention”.

[0164] Although it depends on the type of the subject magnetic recordingmedium, the thickness of the magnetic layer to be formed is preferably 4nm to 1 μm, more preferably 4 nm to 100 nm.

[0165] The magnetic recording medium of the invention may compriseanother layer, if necessary, in addition to the magnetic layer. Forexample, in the case of a disk, a magnetic layer or a non-magnetic layeris preferably formed further on the face opposite to the magnetic layer.In the case of a tape, a back layer is preferably formed on the face ofan insoluble support opposite to the magnetic layer.

[0166] Further, the wear resistance is improved by forming an extremelythin protection film on the magnetic layer and further the slidingproperty is improved by coating a lubricant on the protection film toobtain a magnetic recording medium with sufficiently high reliability.

[0167] As a material for the protection film, oxides such as silica,alumina, titania, zirconia, cobalt oxide, nickel oxide and the like;nitrides such as titanium nitride, silicon nitride, boron nitride andthe like; carbides such as silicon carbide, chromium carbide, boroncarbide and the like; carbon such as graphite, amorphous carbon and thelike can be exemplified and especially preferable one is a hardamorphous carbon generally so called diamond-like carbon.

[0168] The carbon protection film composed of carbon is an extremelythin film having a sufficient wear resistance and scarcely causingbaking in a sliding member, therefore is suitable for material for aprotection film.

[0169] As a method for forming a carbon protection film, a sputteringmethod is generally employed in the case of a hard disk and many methodsemploying plasma CVD with a higher film formation rate have beenproposed for the products such as a video tape and the like whichrequire continuous film formation. Accordingly, these methods arepreferably employed.

[0170] Among them, it is reported that a plasma injection CVD (PI-CVD)method has an extremely high film formation rate and is capable ofproviding a carbon protection film which is hard, has few pin holes, andis excellent as a protection film (for example, JP-A Nos. 61-130487,63-279426 and 3-113824).

[0171] The carbon protection film preferably has Vickers hardness of1,000 kg/mm² or higher, more preferably 2,000 kg/mm². Further, itscrystal structure is preferably an amorphous structure. And theprotection film is preferably non-conductive.

[0172] In the case a diamond-like carbon film is used as a carbonprotection film, the structure can be confirmed by Raman scatteringspectroscopy. That is, in a case a diamond-like carbon film is measured,the structure can be confirmed by detection of a peak at 1,520 to 1,560cm⁻¹. If the structure of the carbon film is shifted from thediamond-like structure, the peak detected by the Raman spectrometry isshifted from the foregoing range and the hardness as a protection filmis lowered.

[0173] As a carbon raw material for forming the carbon protection film,carbon-containing compounds, for example, alkanes such as methane,ethane, propane, butane and the like; alkenes such as ethylene,propylene and the like; alkynes such as acetylene and the like arepreferably used. Further, if necessary, a carrier gas such as argon andan addition gas such as hydrogen, nitrogen and the like for improvingthe film quality may be added.

[0174] If the film thickness of the carbon protection film is thick, theelectromagnetic conversion property is deteriorated and the adhesionstrength to the magnetic layer is decreased. And if the film thicknessis thin, the wear resistance becomes insufficient. Accordingly, the filmthickness is preferably 2.5 to 20 nm, more preferably 5 to 10 nm.

[0175] Further, in order to improve the adhesion strength between theprotection film and the magnetic layer to be a substrate, it ispreferable to previously etch the surface of the magnetic layer with aninert gas or to carry out surface modifying by exposing the magneticlayer to a reactive gas plasma such as oxygen.

[0176] The magnetic layer may be formed to have a layered structure toimprove the electromagnetic conversion property or may have a knownnon-magnetic under layer and intermediate layer thereunder. In order toimprove the running durability and corrosion resistance, as describedabove, a lubricant or a rust-preventing agent is preferably supplied tothe foregoing magnetic layer or the protection film. As the lubricant tobe supplied, known hydrocarbon-type lubricants, fluorine-typelubricants, and extreme pressure agent and the like can be used.

[0177] Examples of the hydrocarbon-type lubricants include carboxylicacids such as stearic acid, oleic acid, and the like; esters such asbutyl stearate and the like; sulfonic acids such as octadecylsulfonicacid and the like; phosphoric acid esters such as monooctadecylphosphate; alcohols such as stearyl alcohol, oleyl alcohol, and thelike; carboxylic acid amides such as stearic acid amide; and amines suchas stearylamine and the like.

[0178] The examples of the fluorine-type lubricants include lubricantsobtained by substituting some or all of the alkyl groups of theforegoing hydrocarbon-type lubricants with fluoroalkyl groups orperfluoropolyether groups.

[0179] The perfluoroether groups include perfluoromethylene oxidepolymers, perfluoroethylene oxide polymers, perfluoro-n-propylene oxidepolymers (CF₂CF₂CF₂O)_(n), perfluoroisopropylene oxide polymers(CF(CF₃)CF₂O)_(n), or their copolymers.

[0180] Further, compounds which are the hydrocarbon-type lubricantshaving polar functional groups such as hydroxyl groups, ester groups,carboxyl groups and the like in the terminals of the alkyl groups and inthe molecules are effective in lowering the friction force, thus aresuitable.

[0181] The molecular weights of them are preferably 500 to 5,000, morepreferably 1,000 to 3,000. If it is less than 500, the evaporationproperty becomes high and the lubricating property becomes low in somecases. Further, if it exceeds 5,000, since the viscosity becomes high, aslider and a disk easily stick to each other to result in occurrence ofrunning stoppage and head crush.

[0182] As the perfluoro ethers, those in trade name of FOMBLINmanufactured by Ausimont K.K., KRYTOX manufactured by Du Pont K.K. andthe like are commercialized.

[0183] Examples of the extreme pressure agents include esters such asphosphoric acid esters such as trilauryl phosphate, phosphorous acidesters such as trilauryl phosphite, thiophosphorous acid esters such astrilauryl trithiophosphite, thiophosphoric acid and the like;sulfur-type extreme pressure agents such as dibenzyl disulfide and thelike can be exemplified.

[0184] The foregoing lubricants can be used alone or in combination of aplurality of them. The methods for coating these lubricants on themagnetic layer or the protection film may involve steps of dissolvingsuch a lubricant in an organic solvent and coating the solution on thelayer by a wire bar method, a gravure method, a spin coat method, a dipcoat method, and the like, or depositing the lubricant on the layer by avacuum evaporation.

[0185] Examples of the rust-preventing agents includenitrogen-containing heterocyclic compounds such as benzotriazole,benzimidazole, purine, pyrimidine and the like and their derivativesobtained by introducing alkyl side chains into the mother cores of them;nitrogen- and sulfur-containing heterocyclic compounds such asbenzothiazole, 2-mercaptobenzothiazole, tetrazaindene cyclic compounds,thiouracyl compounds and the like and their derivatives.

[0186] As described above, in the case the magnetic recording medium isa magnetic tape, a back coat layer (a backing layer) may be formed onthe face of a non-magnetic support where the magnetic layer is notformed. The back coat layer is a layer formed by coating a coatingmaterial for back coat layer formation obtained by dispersing granularcomponents such as an abrading material, an antistatic agent and thelike and a binder in a known organic solvent, on the face of thenon-magnetic support where the magnetic layer is not formed.

[0187] As the granular components, a variety of inorganic pigments andcarbon black may be used and as the binder, nitrocellulose, phenoxyresin, vinyl chloride-based resin, polyurethane type resin may be usedsolely or in combination.

[0188] Further, a known adhesive layer may be formed on the face to becoated with an alloy particle-containing solution and the face where theback coat layer is to be formed.

[0189] The magnetic recording medium produced in such a manner has ancenterline average of the surface in the range of preferably 0.1 to 5nm, more preferably 1 to 4 nm, with a cut-off value of 0.25. That isbecause it is preferable for the magnetic recording medium for highdensity recording to make the surface have extremely excellentsmoothness as described above.

[0190] As a method for obtaining such a surface, a method for carryingout calendering treatment after the magnetic layer formation can beexemplified. Further, varnishing treatment may be carried out.

[0191] The obtained magnetic recording medium may be properly punchedout by a punching apparatus or cut into a desired size by a cuttingmachine or the like, and used.

EXAMPLES

[0192] Hereinafter, the present invention will be described in detailsalong with examples, however the invention is not limited to theseexamples.

Example 1

[0193] (Production of FePt Alloy Particle)

[0194] The following steps were carried out in highly pure N₂ gas.

[0195] A reverse micelle solution (I) was prepared by adding and mixingan alkane solution containing 10.8 g of Aerosol OT (produced by WakoPure Chemical Industries, Ltd.), 80 ml of decane (produced by Wako PureChemical Industries, Ltd.), and 2 ml of oleylamine (Tokyo Kasei KogyoCo., Ltd.) to and with an aqueous reducing agent solution containing0.76 g of NaBH₄(produced by Wako Pure Chemical Industries, Ltd.)dissolved in 16 ml of water (deoxygenation: 0.1 mg/L or lower).

[0196] A reverse micelle solution (II) was prepared by adding and mixingan alkane solution containing 5.4 g of Aerosol OT and 40 ml of decane toand with an aqueous metal salt solution containing 0.46 g of irontriammonium trioxalate (Fe(NH₄)₃(C₂O₄)₃) (produced by Wako Pure ChemicalIndustries, Ltd.) and 0.38 g of potassium chloroplatinate (K₂PtCl₄)(produced by Wako Pure Chemical Industries, Ltd.) dissolved in 12 ml ofwater (deoxygenated).

[0197] While the reverse micelle solution (I) was stirred at 22° C. at ahigh speed by Omni mixer (manufactured by Yamato Scientific Co., Ltd.),the reverse micelle solution (II) was added in an instant. After 10minutes, while being stirred by a magnetic stirrer, the resultantmixture was heated to 50° C. and aged for 60 minutes.

[0198] After mixed with 2 ml of oleic acid (produced by Wako PureChemical Industries, Ltd.), the mixture was cooled to a roomtemperature. After the cooling, the mixture was taken to the atmosphere.In order to break the reverse micelle, a mixed solution of 100 ml waterand 100 ml of methanol was added, thus water phase and oil phase wereseparated. An alloy particle was dispersed in the oil phase. The oilphase was washed 5 times with a mixed solution of 600 ml of water and200 ml of methanol.

[0199] After that, 1,100 ml of methanol was added to flocculate andprecipitate the alloy particle. After the supernatant was removed, 20 mlof heptane (manufactured by Wako Pure Chemical Industries, Ltd.) wasadded to disperse the particle again.

[0200] Further, precipitation by adding 100 ml of methanol anddispersion by 20 ml of heptane following the precipitation, wererepeated twice. Finally 5 ml of heptane was added to prepare an alloymetal-containing solution containing FePt alloy particle with a massratio 2 (water/surfactant) of water and a surfactant.

[0201] With respect to the obtained alloy particle, the yield, thecomposition, the volume average particle diameter, and the distribution(variation coefficient) were measured to obtain the following results.

[0202] Incidentally, the composition and the yield were measured by ICPmass spectrometry (Inductively coupled plasma spectrometry).

[0203] The volume average particle diameter and the distribution werecalculated by measuring the particles photographed by TEM (transmissionelectron microscope: manufactured by Hitachi Ltd. 30 kV) and processingstatistically.

[0204] The alloy particle for measurement was alloy particle collectedfrom the produced alloy particle solution, sufficiently dried, andheated in an electric furnace.

[0205] composition: FePt alloy with 44.5 at % of Pt

[0206] yield: 85%

[0207] average particle diameter: 4.2 nm

[0208] variation coefficient: 5%

[0209] (Oxidation)

[0210] Vacuum degassing was carried out so as to adjust theconcentration of the alloy particle to be 4% by weight and the resultantalloy particle-containing solution was concentrated. After theconcentration, the pressure was turned back to a normal pressure and inorder to oxidize the alloy particle, oxygen gas was supplied to thealloy particle-containing solution. The solvent evaporated at theoxidation was compensated by adding heptane. After the oxidation, 0.04ml of oleylamine was added per 1 ml of the alloy particle-containingsolution.

[0211] (Annealing)

[0212] To the support obtained by firing the surface of a Si support(thickness: 1 mm) to form SiO₂ to the depth of about 300 nm from thesurface, the alloy particle-containing solution after the oxidation wasapplied by a spin coater. The coating amount was adjusted so that theamount of the alloy particle became 0.5 g/m².

[0213] After the coating, annealing was carried out by heating with atemperature rising rate of 50° C./min in an electric furnace (550 ° C.)under N₂ gas atmosphere for 30 minutes and cooling to a room temperaturewith a temperature decreasing rate of 50° C./min to form a magneticlayer (film thickness: 50 nm) containing the magnetic particle and amagnetic recording medium was produced.

[0214] The flow rate of the N₂ gas at the annealing was adjusted to be200 ml/min. Further, with respect to the alloy particle after oxidation,EXAFS measurement was carried out to find that the bonding lengthbetween Fe and oxygen was 19.7 nm and the number of the bonds of Fe withoxygen was 2.2.

Example 2

[0215] A magnetic recording medium was produced in the same manner asExample 1, except that a gas mixture of oxygen and nitrogen (O₂:N₂=1:1)was used in place of oxygen gas at the oxidation.

[0216] With respect to the alloy particle after the oxidation, EXAFSmeasurement was carried out to find that the bonding length between Feand oxygen was 19.8 nm and the number of the bonds of Fe with oxygen was1.8.

Example 3

[0217] A magnetic recording medium was produced in the same manner asExample 1, except that the air was used in place of oxygen gas at theoxidation.

[0218] With respect to the alloy particle after the oxidation, EXAFSmeasurement was carried out to find that the bonding length between Feand oxygen was 19.9 nm and the number of the bonds of Fe with oxygen was1.5.

Example 4

[0219] A magnetic recording medium was produced in the same manner asExample 1, except that H₂ gas atmosphere was employed in place of N₂ gasatmosphere and the heating temperature was changed to 500° C. at theannealing.

[0220] With respect to the alloy particle after the oxidation, EXAFSmeasurement was carried out to find that the bonding length between Feand oxygen was 19.7 nm and the number of the bonds of Fe with oxygen was2.1.

Example 5

[0221] A magnetic recording medium was produced in the same manner asExample 4, except that H₂ gas atmosphere was employed in place of N₂ gasatmosphere and the heating temperature was changed to 400° C. at theannealing.

Example 6

[0222] A magnetic recording medium was produced in the same manner asExample 4, except that the heating temperature was changed to be 450° C.at the annealing.

Example 7

[0223] A magnetic recording medium was produced in the same manner asExample 6, except that the H₂ gas flow rate was changed from 200 ml/minto 600 ml/min and the retention time at 450° C. was changed from 30minutes to 10 minutes at the annealing.

Example 8

[0224] A magnetic recording medium was produced in the same manner asExample 6, except that the H₂ gas flow rate was changed from 200 ml/minto 1,200 ml/min and the retention time at 450° C. was changed from 30minutes to 5 minutes at the annealing.

Comparative Example 1

[0225] A magnetic recording medium was produced in the same manner asExample 1, except that nitrogen gas was used in place of oxygen gas atthe oxidation.

Comparative Example 2

[0226] A magnetic recording medium was produced in the same manner asExample 1, except that argon gas was used in place of oxygen gas at theoxidation.

[0227] Magnetic particles were scraped out from the magnetic layers ofthe respective magnetic recording media obtained by Examples 1 to 8 andComparative Examples 1 and 2 by a spatula and evaluated in terms of themagnetic property, the volume average particle diameter, and the crystalstructure. The results are shown in the following Table 1.

[0228] The magnetic property measurement (measurement of coercive force)was carried out by evaluating the magnetic layers together withsubstrates under condition of applied magnetic field of 790 kA/m (10kOe) by employing a high sensitive magnetization vector measurementapparatus manufactured by Toei Industry Co., Ltd. and DATA processingapparatus manufactured by the same company.

[0229] The volume average particle diameter was measured by alreadymentioned TEM with 300 kV acceleration voltage.

[0230] The analysis of the crystal structure was carried out accordingto a powder method using a goniometer with tubular voltage of 50 kV,tubular current of 300 mA and CuKα-ray as a radiation source, employingan x-ray diffraction apparatus manufactured by Rigaku Corporation. TABLE1 After annealing Volume average particle Coercive Introduced diameterforce Crystal gas (nm) (kA/m) structure Example 1 Oxygen 5 276.5 (3500Oe) Tetragonal FePt + hematite (partially) Example 2 Oxygen:nitro- 5252.8 (3200 Oe) Tetragonal gen = 1:1 FePt Example 3 Air 5 260.7 (3300Oe) Tetragonal FePt Example 4 Oxygen 5 387.1 (4900 Oe) Tetragonal FePtExample 5 Oxygen 5   250 (3165 Oe) Tetragonal FePt Example 6 Oxygen 5355.5 (4500 Oe) Tetragonal FePt Example 7 Oxygen 5 347.6 (4400 Oe)Tetragonal FePt Example 8 Oxygen 5 331.8 (4200 Oe) Tetragonal FePt Com-Nitrogen 5 15.8 (200 Oe) Cubic FePt parative Example 1 Com- Argon 514.22 (1800 Oe) Cubic FePt parative Example 2

[0231] According to Table 1, in the case of Comparative Examples 1 and2, the obtained magnetic particles still had cubic disorder phase with alow coercive force (Hc), meanwhile the magnetic particles of themagnetic recording media of Examples 1 to 8 subjected to the oxidationwere found having high coercive force.

[0232] That was supposedly attributed to that the oxidation made itpossible to lower the phase transformation temperature to thetemperature lower than those in the case of Comparative Examples.

[0233] Further, in Examples 4 to 8, the annealing was carried out inhydrogen atmosphere, so that high coercive force (Hc) was obtained andthe starting temperature of the transformation could be lowered to thetemperature lower than that in other Examples. Particularly, in Examples7 and 8, the annealing period could be shortened.

Example 9

[0234] A layer containing alloy particle that can form CuAu type hardmagnetic order alloy phase on a support (quartz substrate, thickness:1.25 mm) was formed by RF sputtering method using a sputter targetcomposed of an FePt alloy (Fe/Pt=1 by atomic composition ratio).

[0235] The sputtering conditions were as follows:

[0236] substrate temperature: 450° C.;

[0237] sputtering gas pressure: 50 Pa; and

[0238] target-substrate distance: 95 mm.

[0239] After the foregoing layer was formed, oxidation was carried out.The oxidation was carried out by holding (exposing) each specimen at aroom temperature (25° C.) for 6 hours in air. After that, annealing wascarried out by heating with a temperature rising rate of 50° C./min inan electric furnace (450° C.) under H₂ gas atmosphere for 30 minutes andcooling to a room temperature with a temperature decreasing rate of 50°C./min to form a magnetic layer (film thickness: 50 nm) containing themagnetic particle and a magnetic recording medium was produced.

Example 10

[0240] A layer (about 30 nm) containing alloy particle that can formCuAu type hard magnetic order alloy phase on a support was formed by RFsputtering method using a sputter target composed of an CoPt alloy(Co/Pt=1 by atomic composition ratio). A quartz substrate (thickness:1.25 mm) was used as the support. The sputtering conditions were thesame as those of Example 9. The annealing was carried out in the samemanner as Example 9 to form a magnetic layer and a magnetic recordingmedium was produced.

Example 11

[0241] A magnetic recording medium was produced in the same manner asExample 9, except that the atmosphere of the annealing was changed fromH₂ gas to N₂ gas.

Example 12

[0242] A magnetic recording medium was produced in the same manner asExample 10, except that the atmosphere of the annealing was changed fromH₂ gas to N₂ gas.

Comparative Example 3

[0243] A magnetic recording medium was produced in the same manner asExample 9, except that the oxidation was not carried out. Incidentally,a series of the steps from finishing the sputtering to the production ofthe magnetic recording medium were carried out in N₂ gas for preventingthe oxidation of the alloy particle or the like.

Comparative Example 4

[0244] A magnetic recording medium was produced in the same manner asExample 10, except that the oxidation was not carried out. Incidentally,a series of the steps from finishing the sputtering to the production ofthe magnetic recording medium were carried out in N₂ gas for preventingthe oxidation of the alloy particle or the like.

[0245] The magnetic property evaluation of the magnetic layers of therespective magnetic recording media obtained by Examples 9 to 12 andComparative Examples 3 and 4 together with the substrates in the samemanner as Example 1 and the like. The results are shown in the followingTable 2. TABLE 2 Coercive force Annealing treatment (kA/m) afterOxidation Atmosphere Temperature annealing Example 9 With Hydrogen 450°C.   395 (5000 Oe) oxydation Example 10 With Hydrogen 450° C.   316(4000 Oe) oxydation Example 11 With Nitrogen 450° C.   237 (3000 Oe)oxydation Example 12 With Nitrogen 450° C. 197.5 (2500 Oe) oxydationComparative Without Nitrogen 450° C. 47.4 (600 Oe) Example 3 oxydationComparative Without Nitrogen 450° C.   39.5 (500 Oe)  Example 4oxydation

[0246] As shown in Examples 9 to 12, due to the oxidation, highermagnetic properties (coercive forces) than those of Comparative Examples3 and 4 in which no oxidation treatment was carried out, was obtained.Also as shown in Examples 9 and 10, by employing the hydrogen atmosphereat the annealing treatment, the magnetic property could be furtherimproved while the treatment temperature was kept at a low temperature.

[0247] As described above, the invention can provide a production methodcapable of particularly producing a magnetic particle with hardmagnetism without increasing the temperature at the time of annealing, amagnetic particle produced by the production method, and a magneticrecording medium comprising a magnetic layer containing the magneticparticle.

What is claimed is:
 1. A method of producing a magnetic particleincluding producing an alloy particle that can form a CuAu type or Cu₃Autype hard magnetic order alloy phase, oxidizing the alloy particle, andannealing the particle in a non-oxidizing atmosphere.
 2. A method ofproducing a magnetic particle according to claim 1, wherein theatmosphere for the annealing is a reducing atmosphere, an annealingtemperature is 450° C. or lower, and a retention time is 10 minutes orshorter.
 3. A method of producing a magnetic particle according to claim1, wherein a third element other than elements that can form the CuAutype or Cu₃Au type hard magnetic order alloy phase, is added to thealloy particle as an alloying element.
 4. A magnetic particle producedby a method including producing an alloy particle that can form a CuAutype or Cu₃Au type hard magnetic order alloy phase, oxidizing the alloyparticle, and annealing the particle in a non-oxidizing atmosphere.
 5. Amagnetic particle according to claim 4, wherein the atmosphere for theannealing is a reducing atmosphere, an annealing temperature is 450° C.or lower, and a retention time is 10 minutes or shorter.
 6. A magneticparticle according to claim 4, wherein a third element other thanelements that can form the CuAu type or Cu₃Au type hard magnetic orderalloy phase, is added to the alloy particle as an alloying element.
 7. Amagnetic recording medium comprising a magnetic layer containingmagnetic particles produced by a method including producing alloyparticles that can form a CuAu type or Cu₃Au type hard magnetic orderalloy phase, oxidizing the alloy particles, and annealing the particlesin non-oxidizing atmosphere.
 8. A magnetic recording medium according toclaim 7, wherein the atmosphere for the annealing is a reducingatmosphere, an annealing temperature is 450° C. or lower, and aretention time is 10 minutes or shorter.
 9. A magnetic recording mediumaccording to claim 7, wherein a third element other than elements thatcan form the CuAu type or Cu₃Au type hard magnetic order alloy phase, isadded to the alloy particles as an alloying element.
 10. A method ofproducing a magnetic recording medium including forming on a support alayer containing an alloy that can form a CuAu type or Cu₃Au type hardmagnetic order alloy phase, oxidizing the layer, and annealing the layerin a non-oxidizing atmosphere.
 11. A method of producing a magneticrecording medium according to claim 10, wherein the atmosphere for theannealing is a reducing atmosphere, an annealing temperature is 450° C.or lower, and a retention time is 10 minutes or shorter.